1. Field of the Invention
The invention relates to new composite membranes, processes for their preparation and their use for removing benzenes optionally substituted by lower alkyl radicals, hydroxyl, chlorine or bromine from their mixtures with aliphatic and/or cycloaliphatic hydrocarbons, alcohols, ethers, ketones and/or carboxylic acid esters or from effluent.
Membranes can be used for removal of substance mixtures by permeation. A procedure can be followed here in which, for example, a substance mixture in the liquid phase (feed solution) is brought to one side of the membrane and one substance therefrom, a certain group of substances therefrom or a mixture enriched in the one substance or in he certain group of substances is removed, also in the liquid form, on the one side of the membrane (permeation in the narrower sense). The substance which has passed through the membrane and has been collected again on the other side or the substance mixture described is called the permeate. However, it is also possible to follow the procedure in which, for example, the feed is brought to the one side of the membrane in liquid or gaseous form, preferably in liquid form, and the permeate is removed in the form of a vapour on the other side and is then condensed (pervaporation).
Such permeation processes are useful additions to other processes of substance removal, such as distillation or absorption. Permeation, specifically pervaporation, can be of useful service in particular in the removal of substance mixtures which boil as azeotropes.
2. Description of the State of the Art
There have previously been many attempts to adapt membranes of various polymer materials to individual specific purposes. It is thus known from U.S. Pat. No. 2,953,520 to enrich benzene in the permeate and in this way substantially to separate it off from an azeotropic benzene/methanol mixture with the aid of a non-porous plastic membrane of polyethylene. It is furthermore known from U.S. Pat. No. 3,776,970 to separate the two aromatic compounds styrene and ethylbenzene with the aid of a membrane of certain polyurethane elastomers such that styrene is enriched in the permeate. It is furthermore known from German Patent Specification No. 2,627,629 to remove benzene and alkylbenzenes from aliphatic hydrocarbons, cycloaliphatic hydrocarbons, alcohols, ethers and carboxylic acid esters with the aid of polyurethane membranes.